Irreversibility in the microwave absorption of ceramic Y-Ba-Cu-O

We find that, for ceramic samples of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathrm{x}}$, the microwave absorption in a dc magnetic field H and at a temperature T is different for field cooling than for zero field cooling, provided T is below a field-dependent irreversibility temperature. Since the microwaves probe only a layer at the surface of the sample, the irreversibility line in H-T space determined in this way is a property of the surface layer with a thickness given by the microwave skin depth. When the applied H is removed, we observe a remanent microwave absorption which decays logarithmically with time.